Revert "test/ruby/test_m17n.rb: Run a test under assert_separately"

This reverts commit a698181021c11711dc56b74dacbdebc2895b5384.

It failed on macos for a unknown problem.
```
    1) Error:
  TestM17N#test_object_inspect_external:
  Encoding::ConverterNotFoundError: code converter not found (US-ASCII to UTF-16BE)
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:312:in `encode'
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:312:in `inspect'
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:315:in `inspect'
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:315:in `<main>'
      /Users/runner/work/ruby/ruby/src/test/ruby/test_m17n.rb:299:in `test_object_inspect_external'
```
https://github.com/ruby/ruby/runs/4207871418?check_suite_focus=true

0 files changed, 0 insertions, 0 deletions